There is no sense in passing -Wno-error=header-hygiene
if -Wno-header-hygiene is not passed (gcc case). Thus,
use -Wno-error=header-hygiene only with clang.

Bug: chromium:819294
Change-Id: I01bd9771352ab2e8827f1fdd493eb89bb719dd0f
Reviewed-on: https://swiftshader-review.googlesource.com/c/24428
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>